1 Oral valves interradial if present; 10 tentacles............................................................... 2
- Oral valves radial; 15 tentacles.......................... Ceto cuvieria (Cuvier, 1817) (southern and western Australia)
2 Mid-body tube feet project through dorsal and lateral scales.................................................... 3
- Mid-body tube feet absent dorsally and laterally............................................................ 15
3 “Thorn” ossicles present in body wall and tentacles........................................................... 4
- Body wall and tentacles lacking “thorn ossicles”............................................................ 5
4 Mid-ventral radial series of tube feet present; buttons and rosettes present in dorsal and lateral ossicles, lacking spires on dorsal and lateral multi-layered ossicles; live and preserved colour “black”............................................................................................. Psolidium nigrescens H. L. Clark, 1938 (central New South Wales)
- Lacking mid-ventral radial series of tube feet; buttons and rosettes not present in dorsal and lateral ossicles, calcareous spires present on dorsal and lateral multi-layered ossicles; live and preserved colour not “black”................................................................................... Psolidium parmatum (Sluiter, 1901) (NW Australia slope)
5 Dorsal and lateral scales with spires/pillars (not lumps)........................................................ 6
- Dorsal and lateral scales lacking spires/pillars, some species with surface lumps on the scales......................... 7
6 Dorsal and lateral scales predominantly multi-layered ossicles, each with up to two thick, spinous spires, distally digitiform; lacking mid-ventral radial series of tube feet........................ Psolidium oloughlini sp. nov. (southern Australia)
- Dorsal and lateral scales lacking multi-layered ossicles, single-layered, thick, perforated plates with predominantly single, marginal, digitiform spires; mid-ventral radial series of tube feet present............................................................................................ Psolidium spinuliferum (H. L. Clark, 1938) (N and W Australia)
7 Body wall with cupped crosses and/or cups................................................................. 8
- Body wall lacking cupped crosses and/or cups.............................................................. 14
8 Cupped crosses and/or cups in sole of 2 size ranges........................................................... 9
- Cupped crosses and/or cups in sole of 1 size............................................................... 10
9 Rosettes present in dorsal body wall and tentacles; smaller cupped crosses and cups up to 32 μm long; sole with irregular thick perforated plates, knobbed on surface and margin........... Psolidium marshae O'Loughlin & Maric, 2008 (SW Australia)
- Rosettes absent from dorsal body wall and tentacles; smaller cupped crosses and cups up to 24 μm long; sole with smooth per-
forated plates............................................. Psolidium minutum (H.L. Clark, 1938) (E Tasman Sea) 10 Dorsal and lateral cups shallow, completely covered by fine spinelets, including the cross; lacking mid-ventral radial series of tube feet................................. Psolidium mccallumae O'Loughlin & Maric, 2008 (Western Australia slope)
- Dorsal and lateral cups and cupped crosses not shallow; spinelets on rim of cup or distally on branches of cupped cross, not on cross; mid-ventral radial series of tube feet present.......................................................... 11
11 Body wall with predominantly cups, fewer cupped crosses.................................................... 12
- Body wall with predominantly cupped crosses, fewer cups.................................................... 13
12 Knobbed plates in sole; tentacle rosettes small, up to 40 μm long; body rounded ventrally in transverse section............................................................ Psolidium berentsae O'Loughlin & Maric, 2008 (NE Queensland)
- Perforated plates in sole predominantly smooth; tentacle rosettes large, up to 80 μm long; body flat ventrally............................................................... Psolidium laperousazi O'Loughlin & Maric, 2008 (SE Australia)
13 Knobbed perforated plates in sole; irregular perforated plates in tentacles..................................................................................... Psolidium hutchingsae O'Loughlin & Maric, 2008 (N New South Wales)
- Perforated plates in sole predominantly smooth; tentacles lack irregular perforated plates............................................................................................ Psolidium ravum Hickman, 1962 (SE Australia)
14 Lacking series of mid-ventral radial tube feet; dorsal and lateral tube feet inconspicuous in mid-body; dorsal and lateral scales coarsely granuliform............................ Psolidium granuliferum H.L. Clark, 1938 (southern to SW Australia)
- Mid-ventral radial tube feet present as scattered series; dorsal and lateral tube feet conspicuous in mid-body; dorsal and lateral scales finely granuliform............................. Psolidium karenae O'Loughlin & Maric, 2008 (South Australia)
15 Dorsal and lateral scales microscopically smooth; digitiform oral scales interspersed with larger oral scales; very low, flattened profile............................................................ Psolus springthorpei sp. nov. (Queensland)
- Dorsal and lateral scales granuliform; lacking digitiform oral scales interspersed with larger oral scales or valves; low to domed profile....................................................................................... 16
16 Dorsal and lateral scales coarsely granuliform; either cups or half-cylinders present in dorsal ossicles.................. 17
- Dorsal and lateral scales finely granuliform; lacking either cups or half-cylinders in dorsal ossicles.................... 18
17 Macroscopically visible dome-like lumps on dorsal and lateral scales; half-cylinders present but lacking cups in dorsal ossicles................................ Psolus salottii sp. nov. (Great Australian Bight - South Australia to Macquarie Island)
- Macroscopically visible thick granules (not dome-like lumps) on dorsal and lateral scales; cups present but lacking half-cylinders in dorsal ossicles....................................................... Psolus steuarti sp. nov. (Victoria)
18 Little to no encroaching on oral valves by body wall scales; multi-layered ossicles and perforated plates present dorso-laterally, lacking bowl ossicles in sole........ Psolus antarcticus (Philippi, 1857) (Magellan Strait, Macquarie Island, South Georgia)
- Encroaching on oral valves by body wall scales; multi-layered ossicles only present dorso-laterally, bowl ossicles present in sole......................................................... Psolus parantarcticus sp. nov. (Macquarie Island)